The physical and chemical properties of titanium nitride, namely its high electrical conductivity, hardness, melting point and thermal and chemical stability have made it a material of choice for diffusion barrier layers in the manufacture of semiconductor devices. A major challenge in adapting titanium nitride films to sub-half-micron generation ultra-large-scale-integrated ("ULSI") circuit technologies is the synthesis of continuous layers conformal to holes and trenches of sub-quarter-micron diameter in the substrate, corresponding to aspect ratios equal to or greater than 4:1. Such deposition is especially difficult to achieve using lower processing temperatures, such as might be desirable for compatibility with polymer dielectric layers, for example.
Physical vapor deposition ("PVD") techniques, which place the elemental constituents of a deposit onto a substrate, have provided layers satisfactory for very-large-scale-integrated ("VLSI") circuitry. PVD, such as by sputtering, of titanium nitride can be accomplished at temperatures well below 250.degree. C. However, the resulting layer typically has several deficiencies. The most problematic flaw is the poor conformity of the layer to deep and narrow trenches, which renders it unreliable for protection of ULSI contacts from the thermal and chemical rigors of subsequent processing steps.
In chemical vapor deposition ("CVD") techniques, species in the vapor phase over the substrate react to form the deposit on the substrate, often with plasma enhancement. These techniques provide superior coverage of complex topographies, but they generally require higher, sometimes unacceptably high, substrate temperatures during deposition. A variety of organic and inorganic CVD chemistries have been applied to the deposition of titanium nitride. One of the more common of such CVD processes uses a gaseous reactant mixture containing titanium tetrachloride and ammonia. The production of acceptable films from this chemistry typically requires substrate temperatures in excess of 600.degree. C., too high to be universally appropriate for all metallization levels. A lower-temperature variation with titanium tetrabromide still requires a minimum substrate temperature of 400.degree. C.
Thermodynamic calculations indicate that deposition of titanium nitride from a reactant gas mixture of titanium tetraiodide and ammonia could occur at substrate temperatures lower than 300.degree. C. Titanium nitride films have been produced from these reactants in plasma-enhanced CVD processes. (See, e.g., Goldberg et al., pp. 247-257, Advanced Metallization for ULSI Applications in 1994, Blumental et al., eds., Materials Research Society [1995] and Kamata et al., Shinku, Vol. 31, pp. 841-844 [1988].) Reports of this process have described the incorporation of hydrogen in the reactant gas mixture. The hydrogen serves as a carrier gas for the other reactants and is also believed to act as a reducing agent and to prevent the recombination of titanium and iodine species liberated by the thermal decomposition of titanium tetraiodide.
The reported deposition reaction for this chemistry is EQU 2TiI.sub.4(g) +2NH.sub.3(g) +H.sub.2(g) .fwdarw.2TiN.sub.(s) +8HI.sub.(g),
which has a negative Gibbs free energy at temperatures greater than about 283.degree.0 C. However, the properties of titanium tetraiodide preclude its straightforward substitution into an analogous process using lighter halides. Difficulty in delivering titanium tetraiodide into the process chamber presents a significant hindrance to efficient deposition of titanium nitride from this chemistry. The melting point of titanium tetraiodide is about 150.degree. C. At this temperature, the vapor pressure of titanium tetraiodide is only about 1.3 Torr, so that its efficient incorporation into a carrier gas is problematic; this difficulty, in turn, limits the rate at which the tetraiodide can be delivered to the deposition site. Maintaining the transport pathway at an elevated temperature, a common tactic for such nonvolatile reactants, does not enhance the rate of tetraiodide delivery. If heated much above the melting point, titanium tetraiodide partially converts to the triiodide and the diiodide, both of which have even lower vapor pressures than does the tetraiodide.
An alternative to gas-phase transport available for nonvolatile or heat-sensitive reactants is conveyance of the reactant in a liquid state into or into the vicinity of the deposition chamber, and only thereafter volatilizing the reactant. This technique has been used to deposit nitrides from some low-vapor-pressure organometallic titanium compounds. However, the introduction of pure titanium tetraiodide by this direct liquid injection technique would be hindered by its high melting point. Another approach that has been used for reactants having similar problematic physical properties is the injection of a liquid carrier solvent containing the dissolved reactant into the deposition chamber, where both solvent and reactant enter the vapor phase near the substrate upon exposure to the elevated processing temperature. However, finding an appropriate solvent, giving an adequate solubility without the introduction of deleterious contaminants, for a given reactant is not completely straightforward. For example, conventional direct-liquid-injection solvents, such as hydrocarbons or chlorinated hydrocarbons, do not dissolve titanium tetraiodide. Carbon disulfide does dissolve titanium tetraiodide to some extent. However, the reaction of carbon disulfide with ammonia at CVD process temperatures would form byproducts that would be incorporated into the final film product as undesirable impurities.